Refractive correction by laser ablation has evolved into a highly customised procedure in which an accurate topographical aberration map of the patient's eye is obtained, eg. using wavefront techniques, and a precision ablation profile pre-determined to a high degree of dimensional accuracy. The ablation profile is carried out by programming the laser surgical machine to apply multiple successive laser pulses with great precision to the corneal area being treated. The pulses may be of smaller uniform cross-section but scanned over controlled ablation patterns, or of larger cross-section but masked to varying cross-sections, with or without scanning.
Whichever ablation procedure is adopted, there is a requirement that the position of the eye be known initially with great accuracy, and that, during the procedure, any movements of the eye be accurately compensated for in the aiming of the laser pulses. It will be appreciated that patients are awake during the procedure and that movements that may arise include both voluntary and involuntary movements of the eye, and head movements: any of these movements can occur even when, as is normally the case, the patient is holding a steady gaze on a fixation target. Total immobilisation of the eye is not considered practical.
The conventional approach to eye tracking during ophthalmic surgery by ablation has been to focus on the pupil as an object readily detectable in an image or from reflection patterns, and to determine and track the location of the pupil's centre. Examples of this approach are provided by U.S. Pat. Nos. 5,345,281 and 5,980,513, and by international patent publication WO 00/27273, which also cites other references reliant on a pupil-based technique. U.S. Pat. No. 5,980,513 describes a system in which the treatment laser optics are employed to project an infrared sensor beam in multiple spots onto the pupil boundary, and to recover the reflected beam.
It is well recognised that a pupil varies in size with ambient light and other influences, and this is addressed by artificial dilation or by making allowance in the pattern recognition algorithms. However, what is not so well recognised is that the geometrical or mathematical centre of the pupil actually moves by up to 0.7 mm as the pupil expands and contracts in size. These shifts in the pupil centre may have been tolerable in conventional “broad-scale” ablations but are wholly unacceptable in high precision custom ablations. Pupil-based eye tracking is also adversely affected in surgical situations by the fluid changes that occur adjacent the pupil: the required drier environs diminishes the clarity of the pupil boundary.
The present applicant considers that the better reference point for accurate eye tracking is the limbus, the boundary between the iris and the sclera regions, because the limbus maintains a fixed relationship and a close circularity with respect to the lens of the cornea, which is of course the object of the ablation. There have been a number of patents that propose limbus-based eye tracking or position detection, including U.S. Pat. Nos. 5,865,832, 5,966,197, 6,095,648, 6,179,422, 6,299,307, 6,604,825, and 6,702,809, and US patent publication 2002/0013575. These arrangements typically involve detection of an intensity difference between light reflected from the sclera, which is of course white, and the iris, which is coloured.
It is not to be inferred that, by referring to or discussing herein specifically identified documents by number, the applicant is suggesting that these documents constitute common general knowledge.
U.S. Pat. Nos. 5,865,832, 5,966,197, and 6,702,809 disclose eye tracking systems in which the limbus is statically illuminated by lateral light sources, and a lune-shaped image of the whole limbus is projected onto a multiple element detector system. The system of U.S. Pat. No. 5,966,197 employs pairs of detectors on a pair of mutually orthogonal diameters to detect the two limbus positions on each diameter, by monitoring spatially for steps on the detected image.
U.S. Pat. No. 6,179,422 employs a different approach: instead of static illumination of the whole limbus, an illuminating beam is scanned radially across a segment of the limbus, using the same scanning optics as for the ablation beam. The scattered beam is recovered by separate optics and directed to a photo-detector that monitors for an amplitude step indicative of the limbal boundary.
An emerging challenge for eye tracking systems in refraction correction surgery by laser ablation is to match the dynamic capabilities of the ablation process in terms of both responsive times and spatial accuracy. Pulse rates of 300 Hz are now being achieved, for which a tracking response rate of the order of 1 kilohertz is desirable. Higher response rates, e.g. tens of kilohertz, may be sought in future.
It is an object of the invention to provide improved methods for determining and/or tracking the position of an eye, that are capable of the response times and spatial accuracy required for modern ophthalmic laser ablation surgery.